Skin
Skins are graphic files in The Sims,The Sims 2, and The Sims 3 which are used to represent the head, body and clothing of a Sim, but how they work in the three games is somewhat different. The wide availability of third-party skins has unquestionably contributed to the high visibility of the series.
The Sims
Skins in The Sims are comprised of two parts per Sim: a head and a body. Both are simple bitmap images, numbered to correspond with the mesh file which gives them the desired shape on the Sim. Each image file is that Sim's entire body or head, including skin (there are 3 skin tones - light, medium and dark - and it is common for the same outfit to be made in all 3 tones) and hair. However, a Sim's hands are separate, and hands are not part of a body skin, though it is possible for a skin to have custom hands. For example, a skin might have white hands so a Sim using it will appear to be wearing white gloves. Some skins have accessories such as hats and glasses, which have separate BMP and mesh files.
True custom skin tones aren't possible, but variations of the basic skin tones are, as long as they're named so that the game engine "sees" them as light, medium, or dark tones. Some creators made "pale" versions of the light skin tone and made them available for use by others.
Skins are available for different modes of dressing: Everyday, Pajamas, Swimwear and Formal. Some jobs also have "uniform" skins. There are also nude skins for when a Sim is using a toilet, using the Vibrate interaction on another Sim or is taking a bath or shower. (a censor blur appears on top of a naked Sim).
The basic body types are "fit", "fat", "skn" (skinny), and "chd" (child). Custom body types are possible, and they can have their own three-letter identifiers. However, if Hot Date or later is installed, clothing racks will see adult Sims with custom body types as "fit". Therefore, buyable pajamas, swimwear, etc., for adult Sims with custom body types must be labeled as being for the "fit" body type, or those Sims will not be able to access it. Likewise, buyable pajamas, swimwear, etc., for children with custom body types must be labeled as being for the "chd" body type.
In The Sims, all information about a skin file is carried in its filename. The first letter identifies the type of skin. Heads are identified with "C" (for Cranium). Everyday skins are identified with "B" (for Body). Pajamas are identified with "L" (for Lingerie). Formal, Swimwear, Winterwear[Vacation] and High Fashion[Superstar] skins are identified by their initial letters, though players should note that hand files also begin with "H". The next three characters are an identifier for the mesh. Maxis always used numbers, but creators could, and often did, use letters. The next two letters identify gender and age, respectively. Gender is identified with "f" (female) or "m" (male), while age is identified with "a" (adult) or "c" (child). For body skins (but not heads), the next three letters identify the body type. Next, there are three letters which identify the skin tone as either light (lgt), medium (med), or dark (drk). This is followed by an underscore and the actual name of the skin. Mesh files do not have skin tone information, but otherwise follow this convention, though there are some naming conventions that only apply to mesh files.
Unlike The Sims 2, The Sims does not distinguish between custom skins and those included with the game. All head skins and Everyday skins are available when aging a baby to a child, aging a child to an adult with the Age of Instant charm, or generating a townie.
Once created, a Sim's appearance is not editable from within the game, except for their clothes, which can be changed by using a dresser. Some third-party utilities let players change a Sim's appearance.
In Unleashed, skins for cats and dogs use a single bitmap image, though there are still separate meshes for pets' heads and bodies. Pet skins use a somewhat different naming system. All pet skins and meshes start with "B" or "xskin-B", which is followed by the three-character identifier and either "cat" or "dog". Pet meshes are identified as body or head by a "tag" on the end of the "xskin-" file's name. For a dog, those are -PELVIS-DOGBODY for the body and -HEAD-DOGBODY-HEAD for the head. Cat meshes use CATBODY instead of DOGBODY .
The Sims 2
With the advent of the full 3d environment in The Sims 2 (See Comparison between The Sims and The Sims 2), skins become more complex. Skins are still in BMP format but are now contained in a "package" file, which can be extracted (and compiled) with the included Body Shop tool.
A Sim's body is now separate from their clothing, and there are now 4 basic skin tones (light, medium-light, medium-dark and dark) with each skin tone's graphic files being divided by age and gender. In addition, custom skin tones are now possible, and many are available as custom content. That Sims' bodies and clothing are separate makes all outfits available to all Sims of the appropriate age and gender, regardless of their skin tone or body shape. It also means it is now possible to create distinguishing marks such as tattoos that are unique to a Sim, although in practice this is not an economical use of drive space.
Hair is also separate, with different colours and styles, and eye colour can also be set. These traits are carried in a Sim's genetics, and passed on to their children. While The Sims 2 generally distinguishes between custom skins and those included with the game, hair is a partial exception. Some hair creators place their creations in their appropriate color bins. Hair that has been "binned" will still be marked as custom content, but it will not be treated as custom hair, and the game may use it when assigning hair to a newly generated Sim.
Sims can change their appearance through a mirror. Hair colour, hairstyle, facial hair (male teens and older only) make-up, glasses and face paint can be changed.
Clothing has also undergone changes. Not only is there far more detail, but when wearing their Everyday clothes the top half of a Sim's outfit can be made separate from the bottom half, or simply be one piece. The modes of dressing are Everyday, PJs, Underwear, Swimwear, Formal, and Gym Clothes/Workout Gear. Seasons added Outerwear, which will usually be worn in cold weather. Unlike The Sims, which used file-naming conventions to identify skins as Everyday, Formal, etc., The Sims 2 uses internal flags. While separate tops and bottoms can only be Everyday, whole outfits can placed in multiple categories. For example, an outfit can be flagged as both Everyday and Gym Clothes.
Pregnant Sims will wear maternity clothes, although this can't be assigned by players. However, there is at least one mod which allows players to use custom outfits as maternity wear, provided that those outfits were created with that in mind.
The Sims 3
In The Sims 3, players can choose from one of the 6 base colors offered and then use the sliders for a more customized skin tone. Custom skins are also available. The base game doesn't include life stages such as aliens and vampires, but it is possible to create Sims which resemble them by using pale green tones and pale blue tones.
